The paper web formed in the wire section is pressed at two points in the line of making paper or paperboard: in the press section, in which water is removed from the wet paper web by pressing, and in the calender, in which the surface is finished by applying pressure on the already relatively dry paper web. Even though the pressing has a different purpose in the press section and in the calender, the web has quite a different dry matter content when it enters these sections, and the technical development of said sections is guided by the phenomena affecting the paper therein, a common feature for both sections is that they both have a nip in which a given pressure affects the web, the pressure being dependent on the force with which the two moving surfaces forming the nip, normally the shell surfaces of two rotating rolls, are pressed against each other from both sides of the web.
For example in a shoe calender, a nip is formed by combining a roll with a soft surface and a hard roll, which nip extends in the longitudinal direction of the machine, and thus the paper web to be calendered has a long retention time in the calender nip. The shell of the roll with a soft surface is made of an elastic flexible belt, and the roll with a hard surface is a metal roll which is a heated roll functioning as a thermoroll bringing heat to the web. The shell of the soft roll is loaded from inside the roll against the hard roll by a loading shoe, and as a result of this, the paper web travelling along the surface of the hard roll is pressed at a given pressure between the surfaces of the soft shell and the hard roll within a long distance in the nip. At the same time, the elastic shell can be compressed in its thickness direction in the area of the nip. The belt forming the shell of the soft roll can be made of a suitable flexible polymer, such as polyurethane, and inside the belt there is a weave that reinforces the same. Thanks to the nip structure, it is possible to attain a good bulk and stiffness for the paper or paperboard, as well as a uniform smoothness of the surface. To sum up, the calender is especially well suited to on-line calendering of printable paper or paperboard grades. Said calender type is called a shoe calender, and it is known by the trademark “OptiDwell”. One further embodiment of the same is described in the international publication WO 99/28551.
The surface temperature of the thermoroll can even exceed 300° C. As a result of this, thermal stress is exerted on the shell of the opposite roll in those areas in which the web is not positioned between the surface of the thermoroll and the roll with a soft surface, i.e. outside the edges of the web. The surface of the shell of the roll heats considerably, if it is pressed against the thermoroll under the loading without the paper web therebetween, and in the worst case this leads to the damaging of the belt. In practice this can be avoided in such a manner that an overwide web is passed through the calender nip, by means of which the direct pressurized contact of the surface of the thermoroll and the shell surface of the opposite roll is prevented on the edges, and after the calender edge strips are trimmed away from the web. As a precautionary measure, it is also possible to continuously monitor the surface temperature of the belt and cool down the belt when necessary.
The process of passing an overwide web through the calender is disclosed as a principle in the Finnish patent 83249 and in the corresponding publication GB-2218434. Here, the web is at least as wide as the widest soft-faced roll in the calender, and from this web the edge strips are trimmed away before the reel-up.
The overwide web causes unnecessary broke in the calender. Similarly, the continuous monitoring and control of the surface temperature of the belt requires separate measurement means and cooling means and a corresponding control system solely for the belt.
On the other hand, when a long nip attained by means of a shoe calender is used for calendering of paper or paperboard, process advantage is gained by means of the nip which is longer than the conventional nip formed by means of a roll with a soft surface and a hard roll. The optimal nip length depends among other things on the processed grade, the running speed, the temperature of the thermoroll and the material of the belt brought over the shoe element and on the linear load used. When one wishes to change the nip length for example when the grade is changed, the shoe element has to be changed.
Long nip presses comprising a shoe element with a concave surface in the press section are, in turn, disclosed in the Finnish patent 98843 and in the corresponding U.S. Pat. No. 5,908,536, in the U.S. Pat. Nos. 5,084,137, 5,262,011, 5,639,351, and in the international publication WO 99/19562. In these publications there is a shoe element inside a hose roll, which shoe element is loaded against the inner surface of a flexible roll shell with a given force, and the web is guided together with one or two press felts through a nip formed between the hose roll and a counter roll.
The Finnish patent 65103 and the corresponding U.S. Pat. No. 4,713.147 disclose a manner in which the location of the centre of gravity of the supporting force of the shoe element can be changed in the machine direction, thus enabling the distribution of the dewatering pressure in the longitudinal direction of the nip to be changed. A corresponding idea is disclosed in the U.S. Pat. No. 4,973,384, in which the location of the shoe with a concave surface also changes in the direction of the periphery of the press roll. Corresponding ideas are presented in the German application publications DE-4344165 and DE-3317457. All aforementioned publications share the characteristic that the aim therein is to change the pressure curve in the longitudinal direction of the nip when the length of the nip remains substantially the same.
U.S. Pat. No. 4,705,602 discloses a shoe which is used in the press section in the aforementioned position, the loading part of the shoe being composed of two distinct parts in the machine direction, which parts can be moved with respect to each other. Between the parts a pressure pocket is formed, which is connected to a pressurized medium chamber. When the parts are shifted by motors in the machine direction, it is possible to change the length of the pressure pocket and thereby the retention time in the nip. The shoe intended for changing the length of the nip in the aforementioned manner becomes complex in its structure.